A projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and another optical characteristic of a projection optical system is disclosed, wherein, when the projection magnification and the symmetric distortion aberration are adjusted, the other optical characteristic of the projection optical system is adjusted so that a change thereof is reduced very small or zero, such that good optical performance of the projection optical system is retained.
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3. A projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and a coma aberration of a projection optical system, said apparatus comprising:
first changing means for changing a first optical parameter of the projection optical system; second changing means for changing a second optical parameter of the projection optical system; and third changing means for changing a third optical parameter of the projection optical system, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the first optical parameter is changed, are Δβ1, ΔSD1 and ΔA1, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the second optical parameter is changed, are Δβ2, ΔSD2 and ΔA2, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the third optical parameter is changed, are Δβ3, ΔSD #30# 3 and ΔA3, respectively, wherein angles each defined by two of three vectors (Δβ1, ΔSD1 and ΔA1), (Δβ2, ΔSD2 and ΔA2) and (Δβ3, ΔSD3 and ΔA3) are not less than 30 deg. and not greater than 150 deg., and wherein the quantities of all the aforementioned changes correspond to those values each being provided by standardizing an amount of an actual change with respect to an amount of a largest change to be produced due to a corresponding one of said first to third changing means.
1. A projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and an optical characteristic different from the projection magnification and the symmetric distortion magnification of a projection optical system, said apparatus comprising:
first changing means for changing a first optical parameter of the projection optical system; second changing means for changing a second optical parameter of the projection optical system; and third changing means for changing a third optical parameter of the projection optical system, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the first optical parameter is changed, are Δβ1, ΔSD1 and ΔA1, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the second optical parameter is changed, are Δβ2, ΔSD2 and ΔA2, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the third optical parameter is changed, are Δβ3, ΔSD #30# 3 and ΔA3, respectively, wherein angles each defined by two of three vectors (Δβ1, ΔSD1 and ΔA1), (Δβ2, ΔSD2 and ΔA2) and (Δβ3, ΔSD3 and ΔA3) are not less than 30 deg. and not greater than 150 deg., and wherein the quantities of all the aforementioned changes correspond to those values each being provided by standardizing an amount of an actual change with respect to an amount of a largest change to be produced due to a corresponding one of said first to third changing means.
4. A projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and a coma aberration of a projection optical system, said apparatus comprising:
first changing means for changing a position of a first lens of the projection optical system in a direction of an optical axis; second changing means for changing a position of a second lens of the projection optical system in a direction of an optical axis; and third changing means for changing a wavelength of exposure light to be incident on the projection optical system, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the position of the first lens in the optical axis direction is changed, are Δβ1, ΔSD1 and ΔA1, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the position of the second lens in the optical axis direction is changed, are Δβ2, ΔSD2 and ΔA2, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the coma aberration, all to be produced when the wavelength of the exposure light is changed, are Δβ3, ΔSD #30# 3 and ΔA3, respectively, wherein angles each defined by two of three vectors (Δβ1, ΔSD1 and ΔA1), (Δβ2, ΔSD2 and ΔA2) and (Δβ3, ΔSD3 and ΔA3 are not less than 30 deg. and not greater than 150 deg., and wherein the quantities of all the aforementioned changes correspond to those values each being provided by standardizing an amount of an actual change with respect to an amount of a largest change to be produced due to a corresponding one of said first to third changing means.
2. A projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and an optical characteristic different from the projection magnification and the symmetric distortion magnification of a projection optical system, said apparatus comprising:
first changing means for changing a position of a first optical element of the projection optical system in a direction of an optical axis; second changing means for changing a position of a second optical element of the projection optical system in a direction of an optical axis; and third changing means for changing a wavelength of exposure light to be incident on the projection optical system, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the position of the first optical element in the optical axis direction is changed, are Δβ1, ΔSD1 and ΔA1, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the second optical element in the optical axis direction is changed, are Δβ2, ΔSD2 and ΔA2, respectively, wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the wavelength of the exposure light is changed, are Δβ3, ΔSD #30# 3 and ΔA3, respectively, wherein angles each defined by two of three vectors (Δβ1, ΔSD1 and ΔA1), (Δβ2, ΔSD2 and ΔA2) and (Δβ3, ΔSD3 and ΔA3) are not less than 30 deg. and not greater than 150 deg., and wherein the quantities of all the aforementioned changes correspond to those values each being provided by standardizing an amount of an actual change with respect to an amount of a largest change to be produced due to a corresponding one of said first to third changing means.
5. A device manufacturing method, comprising the steps of:
printing a device pattern of a reticle onto a wafer by projection exposure with use of a projection exposure apparatus as recited in any one of developing the exposed wafer.
6. A method according to
7. A method according to
8. A method according to
9. A method according to
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This invention relates to a projection exposure apparatus and a device manufacturing method using the same. More particularly, the invention is suitable usable in a step-and-repeat type or step-and scan type stepper which is a microdevice manufacturing apparatus, for satisfactorily correcting a projection magnification error in projecting an electronic circuit pattern of a reticle onto a wafer surface or various aberrations such distortion, spherical aberration, coma or astigmatism, for example, to thereby produce a high precision projection pattern and accomplish production of a high integration device.
In printing apparatuses (aligners) for the manufacture of semiconductor devices such as IC or LSI, a very high assembling precision and a very high optical performance are required.
As regards the optical performance, a matching precision in the registration of a reticle having an electronic circuit pattern with a wafer is particularly important. The size of a pattern used in a reticle is becoming smaller and smaller, and thus the required matching precision becomes higher and higher. As the factors which are most influential to the matching precision, there are a projection magnification error and a distortion error of a projection optical system.
The projection magnification error is a deviation of the projection magnification (lateral magnification) of a projection optical system from a reference value. The distortion error is a deviation of distortion aberration (distortion) of a projection optical system from a reference value. Both of the projection magnification error and the distortion error appear as a difference between a grid point of a projected pattern and a reference grid point desired.
If a projection optical system has a projection magnification error, there appears an error such as shown in FIG. 5 wherein, while the shape of an idealistic grid is unchanged and kept analogous, the size thereof is changed.
If a projection optical system has a symmetric distortion aberration, there appears an error such as shown in FIGS. 6 and 7 wherein the idealistic grid is deformed into a spindle-like shape or a cask-like shape. If this aberration changes to produce a distortion error, the state of deformation also changes.
Generally, the projection magnification error and the distortion error of a projection optical system are corrected by the adjustment of the projection optical system in its manufacturing process or the adjustment of the projection optical system during the setting operation of an exposure apparatus wherein the projection optical system is incorporated. However, they may be changed after the setting, due to the surrounding environment thereof, particularly, the pressure or temperature. Also, a projection optical system absorbs exposure energies during exposure of a wafer and, due to an increase of the temperature of the system, optical parameters such as a refractive index or a shape, for example, may change. This may cause a projection magnification error and a distortion error, and the magnitude of such error may change thereby.
Correction of a projection magnification error and a distortion error in a stepper has been proposed in some documents such as follows.
Japanese Laid-Open Patent Application, Laid-Open No. 81019/1990 shows a projection exposure apparatus wherein the spacing between a reticle (object plane) and a projection optical system as well as the spacing between lenses of the projection optical system are changed to thereby correct a projection magnification error and a distortion error.
Japanese Laid-Open Patent Application, Laid-Open No. 214334/1985 shows an exposure apparatus wherein, based on a remaining chromatic aberration of magnification in a projection optical system, the wavelength of exposure light is changed to adjust a projection magnification.
Japanese Laid-Open Patent Application, Laid-Open No. 30411/1992 shows an exposure apparatus wherein at least one lens group of a projection optical system is moved in an optical axis direction and, also, the wavelength of exposure light is changed, by which a projection magnification error and a symmetrical distortion error are corrected.
Japanese Laid-Open Patent Application, Laid-Open No. 255748/1996 shows an exposure apparatus wherein two or more particular groups of optical elements in a projection optical system are moved in an optical axis direction, by which a projection magnification error and a distortion error are corrected.
U.S. Pat. No. 5,117,255 shows an exposure apparatus wherein a recticle or at least one lens of a projection optical system is moved three-dimensionally, by which a distortion aberration is corrected.
As the projection magnification error or the distortion aberration is corrected, aberrations of a projection optical system are changed thereby. Among these aberrations being changed, the one which has a smallest tolerance and may cause a problem is a coma aberration. If a coma aberration larger than a tolerance is left in the projection optical system, there may occur a phenomenon of a lateral shift of a circuit pattern being projected. Additionally, it may cause "frequency dependency of distortion" in which the quantity of lateral shift depends on the pattern size or the pattern direction. Since it is a lateral shift amount, it directly influences the final registration (total overlay) of plural mask patterns. Thus, it must be controlled precisely.
It is an object of the present invention to provide a projection exposure apparatus and/or a device manufacturing method using the same, by which a high optical performance can be accomplished.
In accordance with an aspect of the present invention, there is provided a projection exposure apparatus having a function for adjusting a projection magnification, a symmetric distortion aberration and another optical characteristic of a projection optical system, said apparatus comprising: first changing means for changing a first optical parameter of said projection optical system; second changing means for changing a second optical parameter of said projection optical system; and third changing means for changing a third optical parameter of said projection optical system; wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the first optical parameter is changed, are Δβ1, ΔSD1 and ΔA1, respectively; wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the second optical parameter is changed, are Δβ2, ΔSD2 and ΔA2, respectively; wherein a change in the projection magnification, a change in the symmetric distortion aberration and a change in the optical characteristic, all to be produced when the third optical parameter is changed, are Δβ3, ΔSD3 and ΔA3, respectively; wherein angles each defined by two of three vectors (Δβ1, ΔSD1, ΔA1), (Δβ2, ΔSD2, ΔA2) and (Δβ3, ΔSD3, ΔA3) are not less than 30 deg. and not greater than 150 deg.; and wherein the quantities of all the aforementioned changes correspond to those values each being provided by standardising an amount of actual change with respect to a an amount of largest change to be produced due to corresponding one of said first to third changing means.
In one preferred form of the present invention, the first changing means may change a position of a first optical element, such as a lens, of said projection optical system in a direction of an optical axis, the second changing means may change a position of a second optical element, such as a lens, of said projection optical system in a direction of an optical axis, and the third changing means may change a wavelength of exposure light, such as excimer laser light, to be incident on said projection optical system.
In one preferred form of the present invention, the other optical characteristic of the projection optical system may be a coma aberration, for example.
In accordance with another aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: printing a device pattern of a reticle on a wafer by projection exposure with use of a projection exposure apparatus such as described above, and developing the exposed wafer.
In one preferred form of the present invention, the projection optical system may have optical elements constituted only by dioptric elements such as lenses, or the projection optical system may have optical elements constituted by a dioptric element such as a lens and a catoptric element such as a mirror.
The wavelength of exposure light incident on the projection optical system may be one of 365 nm, 248 nm, 193 nm, and 157 nm.
The wavelength of exposure light may be changed to a desired value in accordance with a change in shape of an effective light source of an illumination optical system.
The projection magnification and the symmetric distortion aberration may be changed to desired values, respectively, in accordance with a change in shape of the effective light source of the illumination optical system.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.
FIG. 1 is a schematic view of a main portion of a projection exposure apparatus according to a first embodiment of the present invention.
FIG. 2 is a sectional view of a projection optical system of FIG. 1.
FIG. 3 is a flow chart of device manufacturing processes according to an embodiment of the present invention.
FIG. 4 is a flow chart of a wafer process in the procedure of FIG. 3.
FIG. 5 is a schematic view for explaining a projected pattern, resulting from a projection optical system with a projection magnification error.
FIG. 6 is a schematic view for explaining a projected pattern, resulting from a projection optical system with a symmetric distortion aberration.
FIG. 7 is a schematic view for explaining another example of a projected pattern, resulting from a projection optical system with a symmetric distortion aberration.
FIG. 1 is a schematic view of a main portion of a projection exposure apparatus according to a first embodiment of the present invention.
Denoted in the drawing at 1 is an exposure light source which comprises an injection-locked excimer laser, for example. Alternatively, the light source may comprise an excimer laser not injection-locked but having a resonator and a spectral element disposed in the laser for the band-narrowing. Denoted at 2 is an illumination optical system which comprises a collimator lens 20, an optical integrator 21, a condenser lens 22, and a deflecting mirror 23, for example. The illumination optical system 2 serves to illuminate the surface of a reticle 4 uniformly, by which a uniform illuminance distribution of light is provided upon the surface of a wafer 6. The reticle 4 has formed thereon a fine electronic circuit pattern such as an IC or LSI pattern, for example. A projection optical system 5 operates to project the electronic circuit pattern of the reticle 4 onto the wafer 6 surface at a predetermined reduction magnification.
Denoted at 7 is optical characteristic detecting means which comprises photoelectric converting means having a two-dimensional image pickup means (not shown), for example, to be disposed at the position of the wafer 6 in place of the same. In the detecting operation, the circuit pattern of the reticle 4 or a grid pattern for grid inspection is projected upon the image pickup device. On the basis of imagewise data thus supplied by the image pickup device, the imaging state of the pattern image of the reticle is monitored, by which the projection magnification (error), spherical aberration, coma aberration, astigmatism, and distortion aberration (error) of the projection optical system 5 can be detected photoelectrically.
Denoted at 8 is first correcting means. It operates on the basis of an output signal from the detecting means 7, to move predetermined lens groups (e.g., two lens groups), of the lens groups constituting the projection optical system 5, along an optical axis toward respective desired positions. Each lens group comprises one or more lenses.
Denoted at 9 is second correcting means. It operates on the basis of an output signal from the detecting means 7 to change the emission wavelength of the light source 1.
In this embodiment, the light source 1 may comprise a KrF excimer laser, an ArF excimer laser or an F2 excimer laser, for example, having a spectral element, as described, such as a prism, an etalon or a grating, for example. The spectral element may be actuated to change its posture, by which the emission wavelength of the light source 1 can be changed. The wavelength of exposure light can be thus changed.
In this embodiment, appropriate two lens groups of the lens groups of the projection optical system may be moved along the optical axis to adjust their positions on the optical axis and, additionally, the emission wavelength of the light source 1 may be changed to adjust the value thereof. On the basis of these adjustments, the projection magnification and the distortion aberration of the projection optical system as well as other aberrations can be adjusted. Through this adjustment operation, the projection magnification error and the distortion error of the projection optical system can be corrected satisfactorily, without causing a substantive problem in connection with other aberrations.
The first correcting means 8 of this embodiment comprises lens driving means. As for such lens driving means, a movable barrel for holding lens groups may be guided by an air bearing guide, and the barrel may be moved in an optical axis direction by air pressure from a driving pressure source. Alternatively, a suitable voltage may be applied to a piezoelectric device to move a movable lens barrel.
In this embodiment, the pattern image formed upon the image pickup device may be image-processed by the detecting means 7, and an analysis of the pattern image may be performed thereby. On the basis of the result of the analysis, a signal corresponding to the driving amount for the lens groups may be supplied to the first correcting means 8. Additionally or alternatively, a signal for changing the emission wavelength may be applied to the correcting means 9. A further alternative method may be that the surrounding temperature, pressure and humidity, for example, of the projection optical system is detected by using respective sensors. On the basis of the detected changes, the lens groups may be actuated by the correcting means 8, or the wavelength may be changed by the wavelength changing means 9.
Next, the adjustment of the projection magnification, distortion aberration and other aberrations of the projection optical system in this embodiment, will be described more specifically.
In this embodiment, the adjustment comprises "moving a first lens group in an optical axis direction", "moving a second lens group in the optical axis direction", and "changing the exposure wavelength". Thus, at least three factors are changed. The adjustment can be done in two ways.
The first method is that, upon correction of the projection magnification error and the distortion aberration, at least one of the other aberrations (particularly, coma distortion change) is reduced to zero.
The second method is that, upon correction of the projection magnification error and the distortion error, at least two of the other aberrations (particularly, coma distortion change and image field curvature change) are reduced to a minimum.
A projection exposure apparatus according to this embodiment of the present invention may have two operation modes for performing these two ways of adjustment, selectively, in accordance with the selection by an operator.
In this embodiment, as described, the projection magnification error and the distortion error are corrected and, additionally, a change in another aberration or aberrations is positively corrected or reduced to a minimum.
In place of driving two lens groups, a similar adjustment may be done by changing the refractive index of a gas between lens elements, such as by changing the pressure or temperature thereof.
The first and second methods of adjustment described above will now be explained in greater detail.
Here, it is assumed that, when a certain optical unit Gi of a projection optical system is moved in an optical axis direction by a certain driving amount (length) Si, it produces:
a change in projection magnification: Δβi . . . (2-4)
a change in distortion aberration: ΔSDi . . . (2-2)
a change in spherical aberration: ΔSAi . . . (2-3)
a change in coma aberration: ΔCM1 . . . (2-4)
a change in image field curvature: ΔFCi . . . (2-5)
Also, it is assumed that, when the wavelength (emission wavelength) of the exposure light is changed by a small amount W, it produces:
a change in projection magnification: Δβw . . . (3-1)
a change in distortion aberration: ΔSDw . . . (3-2)
a change in spherical aberration: ΔSAw . . . (3-3)
a change in coma aberration: ΔCMw . . . (3-4)
a change in image field curvature: ΔFCw . . . (3-5)
When two groups of optical units G1 and G2 are moved in the optical axis direction by distances S1, and S2, respectively, and, additionally, the wavelength is changed by W, the quantities of changes of these aberrations, when expressed without subscripts, satisfy the following relations:
Δβ=Δβ1.S1 +Δβ2.S2 +Δβw.W . . . (4-1)
ΔSD=ΔSD1.S1 +ΔSD2.S2 +ΔSDw.W . . . (4-2)
ΔSA=ΔSA1.S1 +ΔSA2.S2 +ΔSAw.W . . . (4-3)
ΔCM=ΔCM1.S1 +ΔCM2.S2 +ΔCMw.W . . . (4-4)
ΔFC=ΔFC1.S1 +ΔFC2.S2 +ΔFCw.W . . . (4-5)
Upon correction of the projection magnification error and the distortion error, the quantity of one of the other aberrations such as coma aberration, for example, is reduced to zero (or to any arbitrary small value). The procedure is essentially the same with the case of the spherical aberration or the curvature of image field.
As the two units move by distances S1 and S2, respectively, and the wavelength is shifted by W, the quantities of changes of β, SD and CM can be expressed by linear equations (4-1), (4-2) and (4-4), respectively.
Δβ=Δβ1.S1 +Δβ2.S2 +Δβw.W . . . (4-1)
ΔSD=ΔSD1.S1 +ΔSD2.S2 +ΔSDw.W . . . (4-2)
ΔCM+ΔCM1.S1 +ΔCM2.S2 +ΔCMw.W . . . (4-4)
By solving these equations, the driving amounts S1 and S2 as well as the wavelength shift amount W can be determined. Since there are three equations with respect to three variables (S1, S2 and W), the solution is a definite one. Namely, the driving amounts S1 and S2 as well as the wavelength shift amount W for performing the corrections by desired correcting amounts Δβ, ΔSD and ΔCM are determined. Also, the quantities of changes of the other aberrations (in this example, the spherical aberration and the image field curvature) can be given by substituting S1, S2 and W into equations (4-3) and (4-5).
Namely, in addition to β and SD, another parameter (coma aberration, in this example) can be adjusted to a suitable value.
The change in focus position of the projection optical system caused by changing the parameters can be compensated for by moving a Z stage, carrying and holding the wafer, in the optical axis direction. To this end, data for focus position changes with a variation in respective parameters may be stored in the exposure apparatus in the form of a table, and the stage may be moved by an amount of a focus position change as can be calculated in accordance with the table. Alternatively, the focus position of the projection optical system may be measured just after the parameter change, by using a TTL autofocus measuring system incorporated into the exposure apparatus, and the Z stage may be moved in accordance with the result of measurement to move and place the wafer to and at the focus position.
The first method of adjustment described above provides a significant advantage when used as follows.
First, a change in coma aberration produced as a result of adjustment of the projection magnification error and of the distortion aberration is corrected completely to zero. As described hereinbefore, the tolerance for the coma aberration is becoming very strict, and this is very advantageous. In this case, the above-described equations may be solved while taking ΔCM+0.
Another example is that the method is used for adjustment of coma aberration when the illumination mode of a projection optical system is changed, among an ordinary or standard illumination, ring-like illumination and quadrupole illumination, for example.
It is known that, when the illumination mode is changed, it produces a change in the projection magnification error and the distortion error. It has been found that the major cause therefor is the coma aberration remaining in the projection optical system. Further, it has been found that, as the illumination mode is changed, it produces a change in an asymmetric error (i.e., coma characteristic) of a resist pattern after being developed. This is also because of the remaining coma aberration.
Further, it has been found that a coma aberration remaining in a projection optical system is a cause for the "frequency dependency of distortion" that the projection magnification (error) and the distortion aberration (error) are variable with the size of an electronic circuit pattern of a reticle. For these reasons, the coma aberration should be suppressed strictly so that it does not remain in the projection optical system.
Even when there is a residual coma aberration in a projection optical system, if adjustment of the coma aberration is attainable, an optimum coma aberration amount can be selected in accordance with an illumination mode selected. Therefore, the projection magnification error, a change in distortion aberration (distortion error) and a change in coma characteristic as the illumination mode is changed, can be adjusted to a minimum.
When the method of the present invention is to be applied to the coma aberration adjustment described above, the aforementioned equations may be solved while taking ΔCM as a desired value (including zero, of course) and Δβ=ΔSD=0. Further, when the projection magnification and the distortion aberration are to be adjusted simultaneously with the coma aberration adjustment, the equations may be solved while substituting desired correction amounts into Δβ and ΔSD.
When the present invention is to be applied to coma aberration adjustment for an illumination mode selected, data for optimum coma aberration amounts corresponding to respective illumination modes may be memorized in an exposure apparatus in the form of a table, and, when the illumination mode is changed, appropriate optical components may be changed in accordance with the information in the stored table.
Alternatively, an aerial image of a reticle device pattern or a test pattern may be observed or measured in response to the illumination made change, or a device pattern actually printed on a wafer may be measured. In place of the above, a developed pattern (resist pattern) based on such a test pattern may be measured. On the basis of such a observation or measurement, a coma aberration may be measured, and it may be corrected in accordance with the produced information to measurement.
Upon correction of the projection magnification error and the distortion aberration, the quantities of changes in two or more of the other aberrations (in this example, coma aberration change and image field curvature change) are reduced to a minimum. Essentially the same procedure will apply to any other combination of two or more aberrations.
From equations (4-1) and (4-2), the driving amounts S1 and S2 can be expressed by using the wavelength shift amount W. Namely,
S1 =A1.W+B1 . . . (5-1)
S2 =A2.W+B2 . . . (5-2)
where
A1 =(Δβ2.ΔSDw -Δβw.ΔSD2)/(Δβ1.ΔSD2 -Δβ2.ΔSD1)
A2 =(Δβ1.ΔSDw -Δβw.ΔSD1)/(Δβ2.ΔSD1 -Δβ1.ΔSD2)
B1 =(Δβ.ΔSD2 -Δβ2.ΔSD)/(Δβ1.ΔSD2 -Δβ2.ΔSD1)
B2 =(Δβ.ΔSD1 -Δβ1.ΔSD)/(Δβ2.ΔSD1 -Δβ1.ΔSD2)
By substituting these equations into equations (4-4) and (4-5), then ΔCM and ΔFC can be expressed as a linear functions of W. Namely,
ΔCM×C1.W+D1 . . . (6-1)
ΔFC×C2.W+D2 . . . (6-2)
where
C1 =A1.ΔCM1 +A2.ΔCM2 +ΔCMw
C2 =A1.ΔFC1 +A2.ΔFC2 +ΔFCw
D1 =B1.ΔCM1 +B2.ΔCM2
D2 =B1.ΔFC1 +B2.ΔFC2
Here, an evaluation quantity ε=(ΔCM2 +ΔFC2) is introduced. Determining the value of W so that this evaluation quantity becomes minimum, then
W=-(C1.D1 +C2.D2)/(C12 +C22) . . . (7)
By substituting this value of W into equations (5-1) and (5-2), the values of S1 and S2 are determined. These values S1, S2 and W are in a combination effective to change the projection magnification by Δβ, to change the distortion aberration by ΔSD and to minimize the square sum of a change in separate aberration. In other words, by using this correction method, the projection magnification and the distortion aberration can be corrected by desired amounts while, an the other hand, changes in two or more aberrations caused by the correction can be suppressed to a minimum.
When the number of aberrations to be minimized is three or more, an equation definition of them can be done in accordance with a similar concept as described above. The change in focus position of the projection optical system as those factors are changed can he compensated for by moving the Z stage, carrying and holding the wafer, in the optical axis direction. This is the same as in the first method of adjustment.
Use of the second method of adjustment described above effectively suppresses changes in other aberrations when the projection magnification error and the distortion aberration are corrected. Therefore, while retaining certain amounts of correction for the projection magnification and distortion aberration, decreases of tolerance for other aberration changes, such as coma, required by miniaturization of a circuit can be fully met. Also, a significant advantage is obtainable with respect to design of a projection optical system.
When this correction method is used, a higher effect is attainable if a combination of optical elements having a close ratio between the quantity of changes in aberrations to be minimized, that is, a close value of ΔCM1 /ΔFCi (i=1, 2, w), is selected. Alternatively, a particular note should be paid so that they come close to values set in the design of the projection optical system.
FIG. 2 is a lens sectional view of the projection optical system 5 according to this embodiment of the present invention. In this embodiment, the projection optical system is aberration corrected with respect to exposure light of a wavelength 248 nm which corresponds to the emission wavelength of a KrF excimer laser.
In FIG. 2, denoted at R is a reticle having an electronic circuit pattern formed thereon. The reticle R is placed on an object plane of the projection optical system. Denoted at G1 -C16 are lenses which constitute the projection optical system.
Denoted at W is a wafer which is placed upon the best image plane of the projection optical system. Denoted at S1 is the distance from the reticle R to the first lens surface of the projection optical system, and denoted at Sk is the distance from the last lens surface of the projection optical system to the wafer W.
Table 1 shows the numerical data of the projection optical system of this embodiment. In this numerical data, Ri is the curvature radius (mm) of the (i)th lens surface in the order from the object side, Di is the spacing (mm) between the (i)th lens surface and the (i)th lens surface in the order from the object side, Ni is the refractive index of the medium between the (i)th lens surface and the (i+l)th lens surface in the order from the object side.
Table 2 shows the quantities of variations in aberrations when, in the projection optical system having the lens data of Table 1, the lens groups G1 and G2 are moved individually in the optical axis direction by 50 microns and also the wavelength is shifted by +10 pm.
| TABLE 1 |
| NA: 0.48 |
| LARGEST OBJECT HEIGHT: 60 mm |
| RETICLE TO 1ST SURFACE: 120 mm |
| EXPOSURE WAVELENGTH: 248 nm |
| R D N |
| G1 2036.785 17.48 1.50841 |
| -233.211 0.50 |
| G2 273.081 19.22 1.50841 |
| -470.487 54.29 |
| G3 153.797 22.01 1.50841 |
| 74.523 18.39 |
| G4 -164.712 28.28 1.50841 |
| -111.304 10.00 |
| G5 -89.910 8.00 1.50841 |
| 116.514 18.90 |
| G6 -69.845 8.00 1.50841 |
| 1182.011 46.87 |
| G7 -410.218 24.00 1.50841 |
| -137.953 0.50 |
| G8 3547.161 25.97 1.50841 |
| -171.895 0.50 |
| G9 371.704 25.13 1.50841 |
| -313.422 0.50 |
| G10 249.898 13.97 1.50841 |
| 986.296 13.48 |
| G11 173.146 49.31 1.50841 |
| 84.594 38.11 |
| G12 -117.818 60.00 1.50841 |
| -209.089 47.32 |
| G13 -77.303 25.32 1.50841 |
| -92.395 0.50 |
| G14 502.276 14.36 1.50841 |
| -326.430 0.50 |
| G15 207.310 14.25 1.50841 |
| -8569.419 0.50 |
| G16 107.980 53.66 1.50841 |
| 271.616 46.83 |
| G17 56.290 18.35 1.50841 |
| 78.166 15.00 |
| TABLE 2 |
| Δβ ΔSD ΔSA ΔCM ΔFC |
| MOVE G1 + 90.4 -65.7 -0.029 -0.006 -0.003 |
| 50 μm |
| MOVE G2 + -12.7 74.9 0.022 -0.022 0.083 |
| 50 μm |
| SHIFT WAVE- -4.7 -20.2 0.044 -0.032 0.014 |
| LENGTH + 10 pm |
| Δβ change (ppm) of projection magnification |
| ΔSD quantity (nm) of change in image deviation at the most abaxial, |
| due to a change in symetric distortion |
| # aberration |
| ΔSA quantity (λ) of change of "value at pupil edge of |
| wavefront aberration on the axis" |
| ΔCM quantity (λ) of change of "the half of the difference |
| between values at opposite edges of the pupil in |
| # meridian direction of wavefront abberation at the most abaxial" |
| ΔFC quantity (μm) of chanqe of "best focus at the most abaxial, in |
| meridian direction". |
In conjunction with the data of Table 2, a conventional system (conventional method) for correcting a projection magnification error and a distortion aberration on the basis of driving two lens groups, will first be considered.
In accordance with calculations made on the basis of the data of Table 2, when the projection magnification is to be corrected by 50 ppm and the distortion aberration is to be corrected by 100 nm, the correction of the projection magnification and distortion aberration can be done if two lens groups are moved respectively by:
G1 :42.2 microns
G2 :103.7 microns
The quantity of changes of the other aberrations are as follows:
Change of Spherical Aberration: 0.021 λ
Change of Comma Aberration: -0.050 λ
Change of Image Field Curvature: 0.170 micron
Thus, it is seen that particularly the coma and the image field curvature change largely.
The two methods described hereinbefore are applied to the present optical system.
The projection magnification and distortion aberration are corrected and, also, the change in coma aberration is reduced to zero.
In accordance with calculations made on the basis of the data of Table 2, when the projection magnification is to be corrected by 50 ppm and the distortion aberration is to be corrected by 100 nm, the correction of the projection magnification and distortion aberration can be done when two lens groups are moved respectively by the following amounts and, also, the emission wavelength is shifted as follows:
G1 :35.9 microns
G2 :81.4 microns
W:31 12.4 pm
Simultaneously, with this adjustment, the change of coma aberration can be suppressed to zero.
The quantity of changes of the other aberrations are as follows:
Change of Spherical Aberration: -0.040 λ
Change of Image Field Curvature: 0.116 micron
Thus, the coma aberration change of -0.05λcaused in the conventional method described above can be removed completely in this embodiment. This embodiment therefore provides can improvement effect of suppressing, to zero, at least one of aberrations remaining after correction of the projection magnification error and the distortion aberration.
While the coma aberration change is corrected to zero in the above-described example, as a matter of course, the coma aberration change can be corrected to any desired value.
Also, while the description has been made with reference to the coma aberration change, correction of spherical aberration or curvature of image field can be done in a similar manner.
As regards changes of aberrations other than the subject of correction to be caused by changing the wavelength as well as changes of aberrations other than the subject of correction to be caused by moving the lens groups, naturally they should be kept small. Generally, therefore, such lens groups may be selected and used in the correction system. Alternatively, in design of the projection optical system, the correction groups are optimized so that changes of aberrations other than the subject of correction are minimized.
If the driving stroke becomes large, inconveniences with respect to machine control may become large. Therefore, the correction system should be structured while taking into account the effect of drives of individual lens groups upon Δβ, ΔSD and ΔCM. Specifically, vectors (Δβ1, ΔSD1, ΔβCM1), (Δβ2, ΔSD2, ΔCM2) and (Δβw, ΔSDw, ΔCMw) having components of the amounts in Table 3, as the (i)th group (where i is a natural number) is driven and the emission wavelength is changed, may be considered, and optimization may preferably be done, in designing, so that the angles defined by these vectors are sufficiently larger than zero deg., preferably, not less than 30 deg. and not greater than 150 deg.
| TABLE 3 |
| Change of Projection Magnification: Δβi, |
| Δβw |
| Change of Distortian Aberration: ΔSDi, ΔSDw |
| Change of Coma Aberration: ΔCMi, ΔCMw |
Also, such group may preferably be selected and used in the correcting system. Particularly, the angles defined by these three vectors at the largest image height are preferably not less than 30 deg. and not greater than 150 deg. While the description has been made with reference to correction of three elements Δβ, ΔSD, and ΔCM, the concept described above can be applied to correction of any three optical performances.
The vector components are magnification, distortion and coma, for example, and there are inconveniences such as varieties in unit, or too large of differences in order of numerals, for example. It is therefore desirable to standardize there vector components with respect to a largest adjustment amount (largest amount of change in the component) which can be adjusted by the correction system.
In the present embodiment, when components constituting the vector are denoted by Δβ, ΔSD and ΔCM, three vectors (Δβ, ΔSD, ΔCM) can be expressed as follows (see Table 2):
G1 Vector: (90.4, -65.7, -0.006)
G2 Vector: (-12.7, 74.9, -0.022)
Wavelength Vector: (-4.7, -20.2, -0.032)
There are too large of differences in digit (figure) of the components above, and these three vectors cannot be placed in space properly. In consideration of it, the component Δβ may be standardized with respect to 50 (ppm) and, similarly, the components ΔSD and ΔCM may be standardized with respect to 150 (nm) and 0.1 (λ), respectively. These numerical values correspond to the largest adjustment amounts to be adjusted by the correction system, and they are determined by the specification of the exposure apparatus, for example. The vectors thus standardized are:
G1 Vector: (1.808, -0.438, -0.060)
G2 Vector: (-0.254, 0.499, -0.220)
Wavelength Vector: (-0.094, -0.135, -0.320)
The angles to be defined by these vectors are:
The angle defined by G1 Vector and G2 Vector: 126.4 deg.
The angle defined by G2 Vector and Wavelength Vector: 82.8 deg.
The angle defined by Wavelength Vector and G1 Vector: 97.9 deg.
Here, these angles can be calculated in accordance with the following equation:
When two vectors V1 and V2 are
V1 =(x1, y2, z1) and
V2 =(x2, y2, z2),
cos θ=[x1 *x2 +y1 *y2 +z1 *z2 ]/[x12 +y12 +z1 +L 2 +L *x22 +y22 +z22 +L ]
The projection magnification and distortion error are corrected while, on the other hand, coma aberration change and image field curvature change are made minimum.
Calculations are made in accordance with the data of Table 2 and equations (5)-(7) and (4). Here, as an evaluation quantity to be minimized, ε'={(0.33*ΔCM)2 +(0.2*ΔFC)2 } is used. It differs from the above-described ε in that coefficients are applied before ΔCM and ΔFC, and they are weight coefficients for performing the minimization while taking balance of the coma aberration and the image field curvature.
In accordance with the calculations, when the projection magnification is to be corrected by 50 ppm and the distortion aberration is to be corrected by 100 nm, the correction of the projection magnification and distortion error can be done when two lens groups G1 and G2 are moved respectively by the following amounts and, also, the emission wavelength is shifted as follows:
G2 : 31.9 microns
G2 : 67.4 microns
W: -20.2 pm
Simultaneously, with this adjustment, the change of coma aberration as well as the change of image field curvature can be minimized. The coma aberration and the image field curvature change by:
Change of Coma Aberration: 0.032 λ
Change of Image Field Curvature: 0.082 micron
Also, the spherical aberration change is as follows:
Change of Spherical Aberration: -0.078 λ
Although there is no remarkable effect of completely removing the change of coma aberration as in the first method described above, as compared with the conventional method, both of the coma aberration change and the image field curvature change are reduced. Thus, even by correction of the projection magnification error and the distortion aberration, a good imaging performance can be retained.
In this case, changes in optical performances other than the projection magnification and distortion aberration as the two lens groups are moved along the optical axis direction should preferably be made small, such that the projection magnification and the distortion aberration are corrected mainly by the movement of two lens groups while a change in any other optical performance caused thereby is minimized by changing the exposure wavelength.
Here, it is now assumed that Δβ1 is the amount of change of the largest image height due to the change in projection magnification and ΔSD1 is the amount of change of the largest image height due to a change in symmetric distortion aberration, both to be produced in response to movement of the first lens group G1, and also that Δβ2 is the amount of change of the largest image height due to the change in projection magnification and ΔSD2 is the amount of change of the largest image height due to a change in symmetric distortion aberration, both to be produced in response to movement of the second lens group. In this embodiment, the projection optical system is so structured that an angle θ that can be expressed as follows is defined to be not less than 30 deg. and not greater than 150 deg.
cos θ=[Δβ1 *Δβ2 +ΔSD1 *ΔSD2 ]/[Δβ12 +ΔSD12 +L *Δβ22 +ΔSD22 +L ]
With this arrangement, the mechanical precision of the movable lens groups as well as their driving strokes can be optimized.
This embodiment uses a projection optical system made of a single glass material and to be used with a KrF excimer laser as an exposure light source (wavelength 248 nm). However, similar correction can be done with the use of a projection lens made of different glass materials. Of course, the exposure light source is not limited to a KrF excimer laser, and the present invention is applicable also to a projection exposure apparatus having an exposure light source of an ArF excimer laser (wavelength 193 nm), an F2 laser (wavelength 157 nm) or an ultra-high pressure Hg lamp (i-line of wavelength 365 nm).
In the correcting methods according to the embodiments of the present invention as described hereinbefore, various types of detecting can be used to obtain the projection magnification error, the distortion error, and changes of coma aberration or image field curvature. Specific examples are as follows:
(a) The surrounding pressure may be measured and, on the basis of a change from a reference state of the pressure, the projection magnification error and the distortion error (aberration) as well as the quantities of changes of the coma aberration and the curvature of image field, for example, may be calculated.
(b) The temperature of the projection optical system may be measured and, on the basis of a change from a reference state of the temperature, the projection magnification error and the distortion error (aberration) as well as the quantities of changes of the coma aberration and the curvature of image field, for example, may be calculated.
(c) The state of exposure/non-exposure may be monitored at a predetermined time interval and, on the basis of the information obtained therefrom, the projection magnification error and the distortion error (aberration) as well as the quantities of changes of the coma aberration and the curvature of image field, for example, may be calculated.
(d) A pattern of a reticle may be printed on a wafer through the projection optical system, and a resist pattern thus produced may be measured. On the basis of it, the projection magnification error and the distortion error (aberration) as well as the coma aberration and the curvature of image field, for example, may be measured.
(e) An aerial image produced by the projection optical system from the pattern of the reticle may be observed and, on the basis of it, the projection magnification error and the distortion error (aberration) as well as the the coma aberration and the curvature of image field, for example, may be measured.
(f) A wavefront aberration of the projection optical system may be measured by using an interferometer, for example, and on the basis of it, the projection magnification error and the distortion error (aberration) as well as the coma aberration and the curvature of image field, for example, may be measured.
Without use of such detecting/measuring means, a correction information table may be stored in a memory of an exposure apparatus, such that the correction may be made on the basis of the correction information. This will be particularly effective in the following case.
Optimum values concerning projection magnifications, distortion aberrations, coma aberrations, and image field curvatures, for example, corresponding to respective illumination modes, may be stored in an exposure apparatus in the form of a table. When the illumination mode is changed by another, correction of these elements may be done in accordance with the information of the table.
Next, an embodiment of a semiconductor device manufacturing method which uses a projection exposure apparatus according to any one of the preceding embodiments, will be explained.
FIG. 3 is a flow chart of a procedure for the manufacture of microdevices such as semiconductor chips (e.g., ICs or LSIs), liquid crystal panels, or CCDs, for example.
Step 1 is a design process for designing a circuit of a semiconductor device. Step 2 is a process for making a mask on the basis of the circuit pattern design. Step 3 is a process for preparing a wafer by using a material such as silicon. Step 4 is a wafer process (called a pre-process) wherein, by using the so prepared mask and wafer, circuits are practically formed on the wafer through lithography. Step 5 subsequent to this is an assembling step (called a post-process) wherein the wafer having been processed by step 4 is formed into semiconductor chips. This step includes an assembling (dicing and bonding) process and a packaging (chip sealing) process. Step 6 is an inspection step wherein an operation check, a durability check and so on for the semiconductor devices provided by step 5, are carried out. With these processes, semiconductor devices are completed and they are shipped (step 7).
FIG. 4 is a flow chart showing details of the wafer process.
Step 11 is an oxidation process for oxidizing the surface of a wafer. Step 12 is a CVD process for forming an insulating film on the wafer surface. Step 13 is an electrode forming process for forming electrodes upon the wafer by vapor deposition. Step 14 is an ion implanting process for implanting ions to the wafer. Step 15 is a resist process for applying a resist (photosensitive material) to the wafer. Step 16 is an exposure process for printing, by exposure, the circuit pattern of the mask on the wafer through the exposure apparatus described above. Step 17 is a developing process for developing the exposed wafer. Step 18 is an etching process for removing portions other than the developed resist image. Step 19 is a resist separation process for separating the resist material remaining on the wafer after being subjected to the etching process. By repeating these processes, circuit patterns are superposedly formed on the wafer.
With these processes, high density microdevices can be manufactured.
While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.
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